In case of a rear wheel drive or four wheel drive vehicles, the power of a vehicle engine is transferred through as transmission and a drive shaft connected to the transmission to differential gears of a vehicle, and the vehicle may be driven based on a rear wheel drive or a four wheel drive by means of the driving force transferred through the differential gears to the rear wheels of the vehicle.
Here, since the torsion stress applied to a drive shaft configured to transfer the driving force of a transmission to differential gears generally concentrates on the surface of the drive shaft, a tubular drive shaft has been widely used in an effort to decrease the weight of a vehicle while maintaining strength.
The method for obtaining a desired structure in a drive shaft is characterized in that one side of a metallic pipe is shrunk, and the other side being opposite to the one side of the metallic pipe is shrunk, and both the shrunk sides are fixed and drawn.
When an end of the drive shaft is shrunk, and the other end being opposite to the end is shrunk, the shrinking portions and the portions where are not shrunk have the same strengths, so the portions where are not shrunk may be bent or transformed in the course of shrinking.
Since it is hard to recycle the drive shaft which has defects took place in the course of shrinking, such defective drive shafts should be discarded.
According to an invention entitled “a method for manufacturing a drive shaft” which was filed on Feb. 14, 2012 and was assigned the application number 10-2012-0014634, and was issued on Nov. 5, 2012 and was assigned the Korean patent number 10-0099797, an inner side surface of the other side of a metallic pipe is heat-treated for thereby forming a strength decrease unit which has a second strength smaller than a first strength of a metallic pipe, and the metallic pipe with the strength decrease unit is shrunk for thereby manufacturing a drive shaft without any strain at an intermediate portion.
According to the above-described Korean registration patent, the shrinking is performed using a plurality of shrinking forms whose diameters gradually decrease.
When the diameter of the metallic pipe with the strength decrease unit is decreased using a plurality of shrinking forms whose diameters gradually decrease, the manufacturing time increases longer than expected. Since a plurality of shrinking forms are used, the shrinking apparatus may need large area.
In addition, when shrinking the pipes with the aid of a plurality of shrinking forms, the force in the axial direction of the metallic pipe may be weakened, for which the axis of the metallic pipe may be distorted or the axes of the shrunk portions and the portions where are not shrunk may deviate from each other.
In addition, since the shrinking is generally performed with the aid of the shrinking forms while applying force in an axial direction of the metallic pipe, over force may be applied in an axial direction of the metallic pipe, so cracks may occur between the shrunk portions and the portions where are not shrunk.